Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270147 AAC

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
847 PM CDT Fri May 26 2017

847 PM CDT

The final showers, along with a couple elevated storms, should
depart the eastern CWA shortly. Attention for the rest of tonight
then turns to fog potential. Apart from the present cloud cover,
it`s a fairly good setup given earlier rain, low temperature/dew
point spreads, and light winds. GOES-16 short wave infrared
channel is indicating some holes in the cloud cover. Difficult to
say how long any will persist or how they will evolve, but it may
not take any lengthy clearing for at least shallow fog to develop.
Patchy fog has been added to the forecast for most outlying
areas. While climatologically not favored in late May, cannot rule
out some pockets of dense shallow fog, maybe most favored in the
southern CWA.

Given the aforementioned uncertainty with cloud cover, it is
possible that Saturday morning starts mostly cloudy, or with fog,
that should erode quickly given the shallow nature of the low-
level moisture.



300 PM CDT

Tonight through Saturday...

For near term convective/severe thunderstorm trends, check for the
latest Mesoscale Discussions above. The weather will quiet down
tonight with the effective warm front shunted well south behind
the MCV. Showers and thunderstorms should exit quickly by the
early to mid evening hours. This will set the stage for a quiet
and pleasant start to the Memorial Day weekend, at least for the
daylight period. Strong heating will take place away from lake
influence on Saturday with plenty of sun followed by increasing
mid and high clouds during the afternoon. Low level thermal progs
support highs in the mid 70s to around 80 away from the lakeshore
and few miles inland, which will be kept in the 60s.



300 PM CDT

Saturday night through Friday...

The effective warm front will gradually lift north in advance of a
likely robust MCS, but mainly stay confined to the southern CWA or
points south. There is uncertainty on timing and trajectory of
this MCS, with the an increasing likelihood for most significant
impacts to remain well south of I-80 or even south of the CWA
counties. Portions of the area are likely to remain for outdoor
activities during the evening hours with likely PoPs for most
widespread shower and thunder coverage after midnight (and highest
thunder coverage along/south of I-80). Deep layer shear and MUCAPE
may be sufficient to support an isolated hail threat and perhaps
localized strong to damaging winds.

A surface low will take shape and drag a cold front across the
area on Sunday with perhaps an isolated risk for gusty to damaging
thunderstorm winds and marginally severe hail southeast of I-55.
There is plenty of uncertainty with the convective scenario on
Sunday and much of the afternoon could certainly be dry northwest
of I-55 or I-57. Expecting breezy west-northwest winds in the
afternoon behind the front, with only modest cool advection so
highs should reach the lower to mid 70s.

Memorial Day Monday could be fairly similar to Sunday temperature
wise. Main feature of note is a large upper low over the northern
Great Lakes, from which some isolated diurnally driven showers
and possibly thunderstorms could form during the afternoon. Again,
most of the day should be dry, so no need to cancel outdoor
plans- just keep an eye on the forecast and the radar. The upper
level low pressure will only slowly slide east through the week,
keeping area under northwest flow aloft and generally slightly
below normal temperatures for the end of May and beginning of
June. Can`t completely rule out any shower activity in the mid to
late week, but it looks to be an overall drier regime.



For the 00Z TAFs...

701 pm...Main forecast concerns are scattered showers this
evening and cigs through Saturday morning.

Scattered showers will likely continue across northeast IL
through mid evening and confidence regarding how long these may
persist is low. Weak low pressure will move across the region
later tonight and mvfr cigs are expected this evening and these
may persist overnight and possibly lower into ifr but confidence
is low and trends will need to be monitored this evening and
through the night.

East/northeast winds around 10kt will slowly diminish later this
evening and possibly turn more northerly overnight...though speeds
will be light. Winds are expected to turn more easterly Saturday
morning and increase up to 10kts Saturday afternoon with a
possible lake breeze. cms


227 PM CDT

Will keep the marine fog advisory going as web cams indicate
haze/fog remains over the open waters.

A high pressure ridge over the lake will continue east. A weak
surface low over the plains will pass south of the lake tonight and
continue across the Ohio Valley Saturday. The low may cause wind
speeds to increase to 10-20 kt over the far southern end of the lake
this evening. One model would suggest gusts up to 25 kt, but do not
have enough confidence to go that high.  High pressure passes over
the lake early Saturday morning and light winds will vary in
direction across the lake.  Variable winds are expected over the
southern end and southerly winds over the northern end.

The next low forms over the plains Saturday and reaches the Great
Lakes region Sunday morning.  The low moves over Michigan Sunday
afternoon and winds become west 10-20 kt behind the low.  The low
over Michigan merges with another Canadian low to the north over
Ontario Sunday night. The low is forecast to remain relatively
stationary over Ontario early next week which may lead to a an
extended period of westerly winds of 15-25 kt. High pressure moves
in behind the low mid to late next week.






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